Plausible that lights go out but engine runs with low battery?

[Stephen Tashiro]

On another forum, a poster complained that an auto shop told her she could drive home after her SUV showed an "alternator not charging" warning. She drove the SUV on the interstate at night. The vehicles' lights went out but the engine kept running. To me, it's an interesting question whether a running engine will continue to run with a battery that is so low that the lights, including the cabin lights, go out.

[Rick Prosser]

I had a car that continued to run after the headlights went out. It was an older model, and I don't think any of the interior lights were on to know if they were dim or not.

The engine ran for a few minutes, but eventually quit also.

[Dan Hintz]

I would consider reaming the owner of that shop a new one for suggesting such a stupid idea. Yes, you can run without an alternator, but only for a few minutes, and the battery will take a severe beating (assuming you can revive it afterwards). Doing so at night on the highway is suicide...

[John Coloccia]

On another forum, a poster complained that an auto shop told her she could drive home after her SUV showed an "alternator not charging" warning. She drove the SUV on the interstate at night. The vehicles' lights went out but the engine kept running. To me, it's an interesting question whether a running engine will continue to run with a battery that is so low that the lights, including the cabin lights, go out.

If the regulator goes, you could potentially be putting something shy of the 14V or so you need to actually charge a battery, but still have plenty to keep the car chugging along. Happens in ATVs all the time. Most will not charge the battery at idle but it will run indefinitely at idle.

[Ken Fitzgerald]

The lights quite possibly have a larger current draw than the engine ignition system requires to keep the enginer running. It is conceivable that the engine could run off a weakened battery while it couldn't put out enough current for the headlights. But...I would not want to drive after dark without headlights.

One snowy December over a decade ago just before Christmas our younest son decided to come home from San Antonio by car. He called me from Glenns Ferry Idaho just of the interstate ...my alternator light is on....the headlights are getting dim....the motor still runs......."What do I do?". I replied get a motel...Long story short he would have been stuck there for a couple days waiting on an alternator to be shipped by bus for his Honda Prelude. I bought one locally, drove it there. A local mechanic with a heated shop installed it while we had breakfast the next morning.

[ray hampton]

your alternator can go bad and your car will still run if your lights or other devices stay off, this happen to me once and if I blow the car horn then the engine would quit, good thing that the car trans was a standard

[Callan Campbell]

All the newest vehicles are tied up in computer networks or CAN BUS stuff , so much so that any system that dies or shuts off due to loss of correct voltage will freak out the rest of the vehicle systems that are struggling to work with whatever is still available. To add to Dan H.'s post, not only does that shop who gave the advice run the risk of killing the battery, ECU's can and WILL get ruined in a low voltage operation issue like this.
I don't know why some can easily survive this and others just go dead and never come back to life once the battery and altn's are working again. Dan and John could probably speak to that. I just know from a daily work stand point that it happens all the time, esp during software downloads or flashing used or new control units while the vehicle is at the shop, and no one is monitoring system voltage or using a constant voltage battery unit to keep the correct voltage in the system while programming.
We as Techs are warned over and over again to keep a minimum voltage level up as we're trying to either talk to the vehicle or program a control unit in it. System crashes and the recovery from them aren't easy or cheap.
Worst one I've seen in the past few years cost 10K in control units[that's one vehicle, lots of dead ECUs:mad:] Just glad it wasn't my fault..:D

[Ken Garlock]

Back some 15 years ago I had an alternator die on me while in transit to a restaurant. Since this was a Buick Le Sabre, the dashboard announced in great detail GET SERVICE NOW. Is it any wonder that Gullible Motorists went casters up?
I made it to the restaurant about 1/2 mile from the place where the definitive dashboard message first appeared. After dinner I cold not get the car to start. Another restaurant patron gave me a jump start. I was then able to get less than 1/2 mile down the road before the care slowed to a crawl. You could count the engine firing. I managed to get the Buick to the edge of the road and called 911. The local police came and helped me get the car off the pavement so as to not block traffic, a concept totally foreign to Texas drivers. The police then called me a tow truck. The next day the diagnosis was a shot alternator.

My lesson learned is that if you think you have ignition system problems, get them fixed THEN. Don't try to to limp it down the road unless the repair shop is down hill and within sight.

The Le Sabre was given to my by my father who had bought it new; he quit driving at age 90.
1. Dad put a new AC evaporator and a heater core on the Buick.
2. I put two heater cores in it.
3. It is now owned by my son, up in Redmond OR. and the Buick needs yet another heater core.

[Van Huskey]

Dumb advice during the day. I have seen cars limp for 10 miles or so BUT as Dan mentions the battery takes a SEVERE whopping and may well be shot after the ordeal.

Idiotic advice at night. With the headlights, marker lights, tail lights etc on you won't get far and what might just be a pain in the day may be a serious hazard at night.

In reality I suppose it could be quite dangerous in the day as well, someone not aware of, or prepared for, the loss of power steering/brakes when the engine dies could be in serious danger even in the day.

[Jeff Monson]

Dumb advice during the day. I have seen cars limp for 10 miles or so BUT as Dan mentions the battery takes a SEVERE whopping and may well be shot after the ordeal.

Idiotic advice at night. With the headlights, marker lights, tail lights etc on you won't get far and what might just be a pain in the day may be a serious hazard at night.

In reality I suppose it could be quite dangerous in the day as well, someone not aware of, or prepared for, the loss of power steering/brakes when the engine dies could be in serious danger even in the day.

Well said Van, as soon as a computer controlled engine looses its minimum voltage it will die out, loss of power steering and brake assist is not a desirable thing.

I guess some people are prone to the "idiot light"

[ray hampton]

do you know about how many cars that are in a crash because of the power steering or power brakes ? , are the newer cars easy to steer when the engine stop ?

[Pat Germain]

do you know about how many cars that are in a crash because of the power steering or power brakes ? , are the newer cars easy to steer when the engine stop ?

If a car is rolling at more than 10MPH, it's relatively easy to steer without power steering. Also, if the engine is running and suddenly dies, there will be enough vacuum remaining in the booster to assist with braking. But after awhile, it will take a lot of force on the brake pedal to make the car stop and steering it at very slow speeds will be difficult.

When a vehicle is running with a dead alternator, it needs very little power to keep the spark plugs firing. The ignition coils take twelve volts and boost it to thousands of volts; but with very low amperage. The ignition system does put a drain on the battery, but it is minimal.

Headlights, on the othe hand, pull many more amps than the ignition system. Thus, a very low battery can power the ignition system but not put out enough amps for the headlights.

A starter requires the most amps of all. This is why if a battery is drained, it typically cannot power the starter, yet the headlights and radio work fine.

[ray hampton]

how do 4 cylinder compare to 6 or 8 cylinders when the engine quit and you had to steer the vehicle [arm strong ] ?are the newer cars equipped with parking brakes ?

[Pat Germain]

how do 4 cylinder compare to 6 or 8 cylinders when the engine quit and you had to steer the vehicle [arm strong ] ?are the newer cars equipped with parking brakes ?

Whether the engine is a 4, 6 or 8 cylinder really doesn't matter when it comes to steering. The weight of the vehicle and the width of the tires do matter. Also, the steering gear would have an affect. For example, sports cars tend to have "quick ratio" steering gears. Those are harder to move without power assist.

And yes, all cars must have still have a mechanical brake/parking brake/emergency brake. But stopping a car with a cable connected to the rear wheels would be more difficult than simply stomping hard on the brake pedal. When you pull hard on a parking brake, you quickly go from almost no affect to locking up the rear wheels. It's hard to "feather" it. At least 80% of stopping power comes from the front brakes. It's a lot harder to stop a car with just the rear brakes.

[ray hampton]

Whether the engine is a 4, 6 or 8 cylinder really doesn't matter when it comes to steering. The weight of the vehicle and the width of the tires do matter. Also, the steering gear would have an affect. For example, sports cars tend to have "quick ratio" steering gears. Those are harder to move without power assist.

And yes, all cars must have still have a mechanical brake/parking brake/emergency brake. But stopping a car with a cable connected to the rear wheels would be more difficult than simply stomping hard on the brake pedal. When you pull hard on a parking brake, you quickly go from almost no affect to locking up the rear wheels. It's hard to "feather" it. At least 80% of stopping power comes from the front brakes. It's a lot harder to stop a car with just the rear brakes.


the steering gear is the main point , do 4 banger engines use the same style of steering gear as the bigger V8 ?

[Pat Germain]

the steering gear is the main point , do 4 banger engines use the same style of steering gear as the bigger V8 ?

Again, the engine size is irrelevant. A sporty car with a four cylinder engine would likely have a quick steering gear. And it would probably have rack and pinion steering. I big SUV is heavier and would have large tires. Thus, the SUV and the sports car could be equally difficult to steer if the engine suddenly died. There are many variables here. But engine size is not one of them.

Other variables would include whether the steering is rack and pinion or a gear with a steering arm, what type of dampener is used (it's like a shock absorber) and road surface. We could probably assume that a Toyota Corolla and a Honda Civic, for example, would be pretty easy to steer if the engine died compared to a sports car or SUV.

Some cars, like the new Shelby Mustangs, have an electric assist for the steering. Shelby Motors did this to improve the fuel economy just enough to get it under the "Gas Guzzler" tax requirements. Typical steering assist is hydraulic which requires a pump that puts a drag on the engine and affects fuel economy. Electrically assisted steering still uses a mechanical connection between the wheel and the suspension. But I don't know how difficult it would be to steer without the assist working.

For many years my daily driver was a '69 Cougar with a modified 351W (V8). For several months, I drove that car when the power steering was out. I can tell you for certain that a '69 Cougar with a V8 is very difficult to steer in a parking lot without steering assist. :-)

[ray hampton]

the reason that I mention v8 is because a bigger engine means a bigger car or truck, the best reason to buy small cars would be the ease of steering them in case of a engine that quit

[Marty Paulus]

The biggest problem is everyone has gotten used to power steering and power brakes. Some of remember manual steering and what that was like. Yes it was tough at slow speed but as the vehicle got moving it wasn't bad. Brakes are another story. They will still work without the engine running as they are a simple hydraulic system. It will take a lot more force on the pedal to get the car to stop however. I am replacing an alternator on my Mom's vehicle today. It started as 'why can't I get my truck to start?' I charged the battery and a day later it still wasn't running. Finally got it started and was on my way to get the remote start looked at when I noticed the volt meter was well below 12. The truck died very quickly when the voltage got below 10. I am hoping to get the rebuilt alternator in today and run it for a while. I am also going to take my smart charger with me to properly recharge the battery. THe standard charger will kill an questionable battery where my smart charger will actually bring it back to life. I have used this on several boat batteries that were all but dead when they were pulled and they have been used for several seasons after that.

And going back to the original question. Yes most vehicles/engines will run with sufficient battery. Look at some forms of racing. Alternators use HP to run so if you can eliminate it you have more power going to the tires. Drag racers rarely use alternators.

[Van Huskey]

For many years my daily driver was a '69 Cougar with a modified 351W (V8). For several months, I drove that car when the power steering was out. I can tell you for certain that a '69 Cougar with a V8 is very difficult to steer in a parking lot without steering assist. :-)

I drove a significantly modified 67 Mustang (also a 351W) for several months without PS back in the day it had 235/50s on the front and it was a bear to steer under 5 MPH!

[Kevin W Johnson]

Back some 15 years ago I had an alternator die on me while in transit to a restaurant. Since this was a Buick Le Sabre, the dashboard announced in great detail GET SERVICE NOW. Is it any wonder that Gullible Motorists went casters up?
I made it to the restaurant about 1/2 mile from the place where the definitive dashboard message first appeared. After dinner I cold not get the car to start. Another restaurant patron gave me a jump start. I was then able to get less than 1/2 mile down the road before the care slowed to a crawl. You could count the engine firing. I managed to get the Buick to the edge of the road and called 911. The local police came and helped me get the car off the pavement so as to not block traffic, a concept totally foreign to Texas drivers. The police then called me a tow truck. The next day the diagnosis was a shot alternator.

My lesson learned is that if you think you have ignition system problems, get them fixed THEN. Don't try to to limp it down the road unless the repair shop is down hill and within sight.

The Le Sabre was given to my by my father who had bought it new; he quit driving at age 90.
1. Dad put a new AC evaporator and a heater core on the Buick.
2. I put two heater cores in it.
3. It is now owned by my son, up in Redmond OR. and the Buick needs yet another heater core.

The heater core problem is likely a victim of another cooling system problem.... an air leak. Its quite likely air is entering the system, which usually get trapped in the heater core, causing corrosion and sludge, which causes the heater core failure.

[Rick Potter]

The alternator died on my daughters '03 Tahoe last week. She was about 50 miles out of Phoenix and pulling a travel trailer. 300 miles from home, in the dark. She made it to a truck stop a couple miles down the road, and spent the night there. A mobile mechanic fixed it the next morning.

Moral: Always tow your house around with you, the alternator always goes out at the worst time possible.

Rick Potter